Various measurement arrangements and measurement methods are known in the prior art, in which the concentration of the gas in a measuring volume is deduced from the absorption of an absorption line of a gas. Basically, these methods may also be used for analyzing gas mixtures. In this case, at least one measurement in an absorption line has to be carried out for each gas. Ideally, each absorption line should be in another range of the light spectrum. Below, we shall confine ourselves to determining the concentration of a single gas, e.g. O2, CO2, He or N2 in air. With respect to gases which naturally occur to a considerable extent in natural air, that is O2 and N2, possibly also CO2, the term concentration refers to the absolutely present amount of gas. Measuring methods used in medical engineering, in which the concentration of the above-mentioned gases is measured, are described inter alia in EP 1772098 A1 (VI11P).
In order to determine the absorption it is possible, for example, to introduce chopped or pulsed light with a highest possible portion in the range of an absorption line of the gas to be measured into a measuring volume by irradiation. By the absorption the gas in the measuring volume is periodically heated, so that initially temperature differences are created, pressure differences as a result thereof, and thus sound. The louder the sound, the higher is the concentration of the gas.
In other measuring methods the light is generated by a broadband light source, and the light transmitted through a measuring volume is measured by a photodiode. This configuration involves the problem that also the light next to the absorption line contributes to the quantum noise in the photodiode. As is known, the quantum noise increases proportionally to the root of the photocurrent. Apart from cross sensitivities with respect to other gases, such a configuration works, therefore, only with an optical filter, which limits the spectral range of the light to the range around an absorption line or an absorption band. In other circumstances the relative alteration of the photodiode current by the absorption is too small. This methods works the better, the less light is permitted by the optical filter to pass through next to the absorption line.
Recently, tunable semiconductor lasers were brought on the market, whose line width is clearly more narrow than the typical absorption line of a gas of interest of about 1 nm, and whose laser line can be varied by more than 1 nm by the intensity of the current flowing through the semiconductor laser. The alteration of the laser line is particularly due to an alteration of the refraction index as a result of a higher temperature of the semiconductor crystal. The influence of the alteration of the refraction index on the wavelength is stronger by about 20 times as compared to the alteration of the volume expansion of the semiconductor crystal at the same temperature change. Such semiconductor lasers are used for analyses in space aviation, especially for expeditions to other planets. In order to determine the absorption of the gas to be measured, the strongly non-linear characteristic of the semiconductor laser has to be subtracted out from the measurement points by a fit.